There is a substantial body of research which demonstrates the beneficial effects of polyunsaturated fatty acid (PUFA) consumption in the prevention and/or treatment of a variety of diseases including cardiovascular conditions, inflammatory diseases and some tumours [1, 2]. Therefore there is a demand for PUFA and their derivatives for use as, or in, dietary supplements and pharmaceuticals.
Separation of unsaturated fats and fat derivatives from saturated fats and fat derivatives is difficult because the unsaturated components are susceptible to thermal and oxidative degradation and because their physical properties do not differ from those of the saturated components [3]. The concentration of the unsaturated components in the form of parent triglycerides is more difficult, because the fatty acids are randomly arranged on the glycerol backbone of the triglyceride [3]. Therefore the parent oil is usually converted into free fatty acids (FFA) or fatty acid ethyl esters (FAEE) before separation into polyunsaturated and saturated fractions is carried out.
The use of urea complexes to separate saturated and monounsaturated fatty acids from polyunsaturated fatty acids has been known since the 1950's [3]. The stability of the complexes formed between urea and FFA is highly dependent on the degree of unsaturation in the fatty acids, with saturated fatty acids forming the most stable complexes and PUFA forming the least stable. The technique has been applied to the concentration of high value all cis-5,8,11,14,17-eicosapentaenoic acid (20:5ω3 or EPA) and all cis-4,7,10,13,16,19-docosahexaenoic acid (22:6ω3 or DHA) from fish oils [3], and to the extraction of γ-linolenic acid from seed oils [4].
The separation procedure is typically performed by dissolving a mixture of FFA (or fatty acid derivatives) in a hot aqueous alcohol solution that contains the appropriate amount of urea. When the solution is cooled, the urea preferentially forms solid complexes with saturated fatty acids and these are removed by filtration. The aqueous alcohol filtrate solution, which is enriched in unsaturated fatty acids, also contains urea. Therefore the fatty acids are recovered from the filtrate by solvent extraction with a non-polar organic solvent, such as hexane or isooctane, in which the urea is insoluble.
The use of non-food grade organic hydrocarbon solvents such as hexane in the extraction of PUFA or derivatives from the filtrate obtained following urea fractionation of a mixture of FFA or fatty acid derivatives is undesirable, particularly where the product is intended for use as a dietary supplement. Loss of the PUFA may occur during the reduction of the hydrocarbon solvent residues to regulatory levels.
Supercritical fluids are selective solvents that have found application in various extraction processes. A supercritical fluid has a density comparable to that of a liquid while exhibiting the diffusion properties of a gas. Thus, supercritical fluids have good solvent properties, which may be varied with pressure and temperature. Carbon dioxide is widely used as a supercritical fluid as its critical temperature and pressure (31° C., 74 bar) are attained relatively easily. Furthermore, CO2 is inert, non-toxic, cheap and readily available.
Lipophilic compounds typically found in fish oil such as FFA, fatty acid esters, squalene, triglycerides, and glycerylethers are soluble to a certain extent in supercritical CO2 [5]. Furthermore, their solubility increases at fixed temperature and pressure when ethanol is added to supercritical CO2 [5]. Urea is almost insoluble in supercritical CO2 and can be precipitated from ethanol solutions when they are admixed with supercritical CO2 [6].
Known methods for employing supercritical fluids in combination with urea to separate PUFA or their derivatives from mixtures with other fatty acids or derivatives are batch-wise processes, with consequent low production rates. The use of supercritical fluid extraction after conventional urea fractionation to separate PUFA from other fatty acids requires the use of organic solvents.
WO01/10809 describes a batch-wise process in which polyunsaturated fatty acid ethyl esters can be recovered from solid complexes of mixed FAEE and urea using supercritical CO2 at high pressures and relatively high temperatures, with the extent of recovery increasing with increasing temperature [7]. No application of this process to liquid feedstocks is described, and the applicants note that the use of co-solvents confers little or no advantage in the extraction of the esters. However, the specification does describe the use of ethanol as a co-solvent which, undesirably, caused urea to precipitate in the valves of the equipment.
In other systems, neat FFA or fatty acid derivatives are dissolved in supercritical CO2 and the mixture contacted with solid urea that absorbs the saturated fatty acids or derivatives [3, 8, 9]. The capacity of the solid for saturated fatty acids is quickly reached, and the urea must be changed before the outlet composition of the extract in the process becomes the same as the original fatty acid material. An alternative method in which esters dissolved in supercritical CO2 are mixed with a solution of urea in ethanol/water has also been described. Some of the esters are absorbed by the solution and form a solid complex with urea. This process has a low throughput, and requires long crystallisation times [3, 9].
Fatty acid esters have been fractionated in packed columns to produce PUFA concentrates using supercritical CO2 after conventional urea crystallisation [10, 11]. However, this method of processing does not eliminate the need for organic hydrocarbon solvents or ensure that hydrocarbon solvent residues can be completely removed.
It is therefore an object of the present invention to provide a process for extracting a lipophilic compound from a solution containing urea, which goes some way towards overcoming the above disadvantages, or at least provides the public with a useful choice.